Mobile communications has been rapidly developed as the main instrument serving the information society. This instrument has been influenced by the developments of two technologies: signal processing using modulation or demodulation of transmitted data over limited frequency bands, and the technology of manufacturing radio frequency (RF) hardware parts.
In particular, filters are most important among the parts used for RF mobile communication devices. Filters are able to select the signal requested by the user from numerous signals on the public communication network, or filter a signal transferred by the user. Thus, excellent filters were previously developed for high quality mobile communication. Recently, higher performance filters have been developed to be thinner and more light-weight. These features ensure that mobile communication devices consume less power and more portable.
In general, a resonator or a frequency filter is the device that transmits the frequencies of a message in a predetermined band, and filters the frequencies on other bands being produced by various electronic devices such as wireless phones, personal communication service devices, cellular phones or devices for the international mobile telecommunications 2000 (IMT-2000) as a band pass filter.
Presently, the dielectric filter and the surface acoustic wave (SAW) filter are widely used as the RF filter for mobile communication devices. The dielectric filter has some advantages such as high permeability, low insertion loss, stability at high temperatures and good mechanical strength. However, the dielectric filter is too large to be applied in a monolithic microwave integrated circuit (MMIC). Though the mono-blocked or the multi-layered surface mounted device (SMD) resonators are now developed with smaller dimensions, SMD resonators do not sufficiently overcome their size problems.
SAW filters are relatively smaller than dielectric filters and have simpler signal processing and more simplified circuits. The SAW filter also can be manufactured using semiconductor technology, and gives high quality results since the SAW filter's side rejection in its pass-band is greater than that of the dielectric filter. However, the SAW filter has large insertion losses below 3 dB and its manufacturing costs are high because it is manufactured using single crystal piezoelectric substrate composed of lithium niobate (LiNbO3) or lithium titanate (LiTaO3). Also, the SAW filter is manufactured using an ultraviolet ray exposure apparatus so that the SAW filter may not be used for high frequency bands above 5 GHz because the line width of the inter-digital transducer (IDT) is above 0.5 μm.
Film bulk acoustic resonators (FBAR) have been developed for next generation mobile communication devices. The FBAR can be mass-produced at low cost using semiconductor technology and is ultra light weight and thin. In addition, the FBAR can be freely combined with RF active devices. In particular, the FBAR has good insertion loss of about 1 to 1.5 dB-smaller than or identical to that of the dielectric filter. The FBAR also has excellent side rejection higher than the of the SAW filter by about 10 to 20 dB, thereby providing high quality results.
At present, the active elements of mobile communications include the Hetero-junction Bipolar Transistor (HBT) or the Metal Semiconductor Filed Effect Transistor (MESFET), but these are gradually being simplified and minimized by monolithic microwave integrated circuit (MMIC) technology. However, passive components of RF technology such as the filter, the duplexer filter or the antenna are relatively large and complicated structures so that the single chip may not be achieved due to passive components.
The FBAR or the stacked thin film bulk wave acoustic resonators (SBAR) are manufactured by forming piezoelectric material such as zinc oxide (ZnO) or aluminum nitride (AIN) on a substrate composed of silicon or gallium-arsenic (Ga—As) using RF sputtering method, thereby achieving the desired resonance provided by the piezoelectric material.
The thin film resonator can be manufactured at low cost and yet provide high quality, making it is applicable for use in various devices with frequency bands of 900 MHz to 10 GHz. In addition, the thin film resonator can be much smaller than the dielectric filter and has the added benefit of an insertion loss smaller than that of the SAW filter. Hence, a thin film filter such as FBAR can be used in any MMIC dependent on high quality and good stability.
The method for manufacturing conventional FBAR or the SBAR is disclosed at U.S. Pat. No. 6,060,818 issued to Richard C. Ruby et al.
FIG. 1 is a cross-sectional view showing the FBAR and FIGS. 2A to 2C are cross-sectional views illustrating the method for manufacturing the FBAR in FIG. 1.
Referring to FIG. 1, the FBAR 10 is formed on a silicon substrate 15 and the FBAR 10 includes a bottom electrode 20, a piezoelectric layer 25 and a top electrode 30.
An oxide layer 35 is formed on the substrate 15 and a pit 40 is interposed between the substrate 15 and the FBAR 10.
Referring to FIG. 2A, the silicon substrate 15 is provided, and then the pit 40 having a predetermined depth is formed on the substrate 15 by partially etching the substrate 15. Subsequently, the oxide layer 35 is formed on the whole surface of the substrate 15 by the thermal oxidation method.
As shown in FIG. 2B, after a sacrificial layer 45 composed of phosphor silicate glass is coated on the oxide layer 35 to fill the pit 40, the sacrificial layer 45 is polished so that the sacrificial layer 40 remains only in the pit 40.
Referring to, FIG. 2C, after the bottom electrode 20 composed of molybdenum (Mo), the piezoelectric layer 25 composed of aluminum nitride (AIN), and the top electrode 30 composed of molybdenum are successively coated on the oxide layer 35 and on the sacrificial layer 45 filling the pit 40, the bottom electrode 20, the piezoelectric layer 25, and the top electrode 30 are patterned. Then, the sacrificial layer 45 is removed using an etching solution containing hydrofluoric acid (HF), thereby completing the FBAR 10 as shown in FIG. 1.
The conventional FBAR is, however, formed on the substrate where the cavity is positioned, giving the FBAR two-dimensional construction. Hence, the conventional FBAR provides poor quality performance with an increased insertion loss.
In addition, the interference of the substrate may not be blocked, causing the power loss of the FBAR to increase. The size of the FBAR is limited also, since the FBAR is formed over the cavity in the substrate in order to receive the deformation of the piezoelectric layer.
Furthermore, the process for etching the silicon substrate demands much time, and the cost of manufacturing the FBAR increases because the conventional FBAR is formed on the silicon substrate where the cavity is positioned.
To overcome such problems, research institutes at Berkeley and Michigan Universities have disclosed a thin film bulk acoustic resonator (TFBAR) with a three-dimensional structure on a substrate using the micro-electromechanical system (MEMS) technology. However, the TFBAR may not be mass-produced and packaging the TFBAR may be difficult since its structure is complicated and the integration device including the TFBAR is difficult.